The present invention relates to the recovery of coal fines as a valuable product and the clarification of water for recycling from coal tailings permitting the use of a single stage unit operation.
A recognized challenge in coal industry is inefficient recovery of coal fines, compounded with the trend of producing more fines with continuous mining systems and subsequent coal preparation operations. Until very recent years, fine coals were merely washed to large volume of tailings stream, not only losing energy resources but also having some environmental consequences. Coal tailings typically contain less than 3% solids and about 60% of the solids are coal. Considering that a large quantity of water has to be treated before recycle or safe discharge to the environment, economics dictate that as much of the marketable coal as possible should be recovered from the tailings and water contained in the tailings be recycled if possible. From both economic and environmental considerations, the incentive to recover fine coal from a tailings stream (and/or use the water) is evident.
A few processes have been tested for recovering fine coals from tailings stream, including froth flotation and oil agglomeration. Although froth flotation featuring with microbubble technology had some success in recovering fine coals, thermal efficiency of the recovered coal is in general low due to the presence of surface moisture, encumbered by large surface areas (see for example NORTON, J., 1984. Finding the Economic Limit of Fine Coal Cleaning, Coal Mining, September, p. 38-40.). In addition, the discharge from flotation circuit remains to contain large amount of mineral matters including fine clays, which are difficult to remove by conventional filtration or thickening.
Oil agglomeration, on the other hand, appears to be promising in recovering fine coals, as there is virtually no lower particle size limit and the ultimate floatability of coal is less affected by size compared to froth flotation (MEHROTRA, V. P., SASTRY, K. V. S. and MOREY, B. W., 1983. Review of Oil Agglomeration Techniques for Processing of Fine Coals, Int. J. Miner. Process. 11, p. 175-201.). An attractive feature of oil agglomeration process is a product of low moisture content. However, the recovered fine coals still need to be pelletized by adding an extra amount of heavy oil and screened to a few millimeters in diameter, to facilitate the storage and handling and to ensure an effective flame contact in combustion chambers. As in froth flotation, a second stage treatment to remove fine particles from process effluent for water recycle is needed with the oil agglomeration process.
An object of the invention is to provide an economic process capable of recovering fine coals and cleaning up process water for recycle, preferably in an integrated, single stage operation. Broadly the present invention related to extracting coal fines from an aqueous phase suspended coal fines and hydrophilic mineral matters comprising mixing an extraction oil with the coal fines suspended in said aqueous phase, the oil being added in an amount effective to extract the coal fines by hydrophobic extraction and form a nonaqueous phase containing said coal fines and said oil and a modified aqueous phase containing said hydrophilic mineral matters, adding at least one of a flocculating agent, a coagulating agent or a combination of said flocculating agent and said coagulating agent to said aqueous phase or said modified aqueous phase to facilitate the separation of said hydrophilic mineral matters from said aqueous phase to form a clarified aqueous phase and disposing of said mineral matters from clarified aqueous phase.
Preferably said mixing of said extraction oil for recovering fine coals and said addition of said at least one of said flocculating agent, said coagulating agent or said combination of the two for clarification of process water are integrated into a single stage.
Preferably said one stage includes removing the separated mineral matters from the aqueous phase to provide said a clarified aqueous phase for recycling.
Preferably said extraction oil will be added in the amount of between about 100 and 250% based on the dry weight of the coal fines in the suspension.
Preferably said extraction oil will be selected from the group consisting of heavy crude, light mineral oils, fuel oils and landfill gas condensates.
Preferably said one comprises said flocculating agent, which is added to a concentration below 30 ppm.
Preferably said flocculating agent is selected from the group comprising cationic or anionic flocculants.
Preferably said one comprises said coagulating agent is added to a concentration of up to about 1000 ppm.
Preferably said coagulating agent is selected from the group comprising positively charged aluminum hydrosols and calcium, magnesium or other multivalent cations.
Preferably said one comprises said combination of said flocculating agent and said coagulating agent is used for separation of said hydrophilic mineral matters from said modified aqueous suspension.
Preferably said flocculating agent will be an anionic flocculant and said coagulating agent will be multivalent cations.
Preferably said flocculating agent and said coagulating agent are mixed in the ratio of between {fraction (1/10)} and {fraction (1/100)} of flocculating agent to coagulating agent.